How this calculator works
Expansion makes present proper recession speed proportional to distance in a homogeneous universe. When estimating speed from a measured redshift, v approximately equals cz only for small redshift. Local peculiar velocities can dominate for nearby galaxies. The entered Hubble constant is a parameter, not a claim of a unique measured value. Its reciprocal is a characteristic time and need not equal the age of the universe.
Formula
v = H0 d; d = v/H0; H0 = v/d; t_H = 1/H0. Convert H0 from km/s/Mpc to 1/s before using SI distances.
Worked example
With H0 = 70 km/s/Mpc, a galaxy 100 Mpc away has a recession speed of 7000 km/s.
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Frequently asked questions
Is Hubble time the age of the universe?
Only in particular expansion histories. Matter and dark energy change the age relative to 1/H0.
Can I use this for a nearby star?
No useful expansion distance follows from a stellar radial velocity. Bound systems and local motions dominate.
Is a large-redshift distance simply cz/H0?
No. A cosmological model is needed to distinguish proper, comoving and luminosity distances.